Circuit and method for contour enhancement

ABSTRACT

A semiconductor integrated circuit includes a differential calculating unit which obtains a differential between a value of a pixel of interest and values of surrounding pixels contained in an image signal supplied from an image sensor, a dead-zone generating unit which defines a predetermined range of pixel values, and a comparison unit which checks whether the differential falls outside the predetermined range, wherein contour enhancement is applied to the pixel of interest in response to a determination by the comparison unit that the differential falls outside the predetermined range.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is based upon and claims the benefit ofpriority from the prior Japanese Patent Application No. 2002-216848filed on Jul. 25, 2002, and Japanese priority application No.2002-316078 filed on Oct. 30, 2002, with the Japanese Patent Office, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to semiconductorintegrated circuits and methods for image processing, and particularlyrelates to a semiconductor integrated circuit and a method for enhancingcontours in image signals supplied from an image sensor.

[0004] 2. Description of the Related Art

[0005] A CMOS image sensor includes a reception section in which aplurality of photo-diodes are arranged in a matrix array. Thesephoto-diodes each correspond to a pixel of a captured image. Incidentlight is subjected to photoelectric conversion on a pixel-by-pixelbasis, and the electric charge obtained by the photoelectric conversionis accumulated in an electric charge accumulation section for reading ofdata. Data of each pixel is supplied to a processing circuit such as animage processor for such processing as RGB conversion, contourenhancement, and format conversion, followed by being supplied to amemory or to an image display apparatus or the like.

[0006] In order to enhance image contours, conventionally, uniform imageenhancement is applied unvaryingly to all the pixels of an image.Namely, differences of image data levels with adjacent pixels areobtained with respect to every pixel of the image, and these differencesare enlarged by a predetermined factor so as to enlarge differences ofimage data levels between adjacent pixels.

[0007] A Japanese Patent Application Publication No. 5-22601 discloses aprocess that avoids an artifact of multiple contours without degradingthe performance of contour enhancement. This process obtains afirst-order derivation by calculating differentials in a predetermineddirection while providing a dead zone in which no processing isperformed, and further obtains a second-order derivation in thispredetermined direction, followed by performing an averaging process togenerate an edge-enhanced signal.

[0008] When all the pixels of image data are subjected to imageenhancement, however, image noise is enhanced altogether, resulting inundesirable degradation of image quality.

[0009] Accordingly, there is a need for a method and a circuit forcontour enhancement that can easily generate contour-enhanced imageswithout degrading image quality.

SUMMARY OF THE INVENTION

[0010] It is a general object of the present invention to provide amethod and a circuit for contour enhancement that substantially obviatesone or more problems caused by the limitations and disadvantages of therelated art.

[0011] Features and advantages of the present invention will bepresented in the description which follows, and in part will becomeapparent from the description and the accompanying drawings, or may belearned by practice of the invention according to the teachings providedin the description. Objects as well as other features and advantages ofthe present invention will be realized and attained by a method and acircuit for contour enhancement particularly pointed out in thespecification in such full, clear, concise, and exact terms as to enablea person having ordinary skill in the art to practice the invention.

[0012] To achieve these and other advantages in accordance with thepurpose of the invention, the invention provides a semiconductorintegrated circuit, including a differential calculating unit whichobtains a differential between a value of a pixel of interest and valuesof surrounding pixels contained in an image signal supplied from animage sensor, a dead-zone generating unit which defines a predeterminedrange of pixel values, and a comparison unit which checks whether thedifferential falls outside the predetermined range, wherein contourenhancement is applied to the pixel of interest in response to adetermination by the comparison unit that the differential falls outsidethe predetermined range.

[0013] In the semiconductor integrated circuit as described above,contour enhancement is not unvaryingly applied to the entirety of theimage data, but is applied to only those image portions which areidentified as requiring contour enhancement. This achieves efficientcontour enhancement that avoids unnecessary enhancement of noise.

[0014] The calculation of the differential and the contour enhancementmay be performed only with respect to the green-color component among aplurality of color components or only with respect to the luminancecomponent. With this provision, a line-delay memory needs to be providedonly for the green-color component or the luminance component, therebyachieving a relatively small circuit size.

[0015] Further, a method of enhancing contours includes the steps ofobtaining a differential between a value of a pixel of interest andvalues of surrounding pixels contained in an image signal supplied froman image sensor, defining a predetermined range of pixel values,checking whether the differential falls outside the predetermined range,and applying contour enhancement to the pixel of interest in response toa determination that the differential falls outside the predeterminedrange.

[0016] Other objects and further features of the present invention willbe apparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a block diagram of an image processor for performingimage processing including contour enhancement according to the presentinvention;

[0018]FIG. 2 is a block diagram of a contour enhancing unit according toa first embodiment of the present invention;

[0019]FIG. 3 is an illustrative drawing for explaining the pixel ofinterest subjected to contour enhancement; and

[0020]FIG. 4 is a block diagram of the contour enhancing unit accordingto a second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] In the following, embodiments of the present invention will bedescribed with reference to the accompanying drawings.

[0022]FIG. 1 is a block diagram of an image processor for performingimage processing including contour enhancement according to the presentinvention.

[0023] The image processor of FIG.1 is implemented as an LSI chip, andincludes a defect correcting unit 11, a RGB conversion unit 12, a whitebalancing unit 13, a contour enhancing unit 14, a gamma correction unit15, and a format conversion unit 16.

[0024] The defect correcting unit 11 receives image signals from animage sensor such as a CCD (charge coupled device), and carries outdefect correction. An image sensor inevitably includes pixels that aredefective and thus cannot properly operate. Such pixels appear as fixedblack dots or white dots. The defect correcting unit 11 is provided forthe purpose of correcting image data of these defective pixels. Theimage signals having undergone the defective pixel correction aresupplied from the defect correcting unit 11 to the RGB conversion unit12. The RGB conversion unit 12 outputs image signals by obtaining colordata of each pixel based on color information of the RGB Bayer array.The image signals output from the RGB conversion unit 12 are thensubjected to white balancing by the white balancing unit 13.

[0025] After this, the contour enhancing unit 14 for performing contourenhancement of the present invention carries out a process for enhancingcontours of the supplied image data. The gamma correction unit 15applies a process for correcting gamma characteristics of the outputapparatus. Finally, the format conversion unit 16 converts the imagesignals into a format acceptable to a subsequent processing stage, andthe converted image signals are output.

[0026]FIG. 2 is a block diagram of the contour enhancing unit 14according to a first embodiment of the present invention.

[0027] The contour enhancing unit 14 of FIG. 2 includes a differentialcalculating unit 21, a dead-zone generating unit 22, a comparison unit23, and an enhancement value generating unit 24. The differentialcalculating unit 21 obtains differentials of pixel values between apixel of interest and surrounding pixels. The dead-zone generating unit22 obtains a predetermined range of pixels that is defined by an upperlimit and a lower limit. The comparison unit 23 checks whether thedifferentials supplied from the differential calculating unit 21 fallinto the predetermined range indicated by the dead-zone generating unit22. The enhancement value generating unit 24 generates a correctionvalue (enhancement value) for correcting the value of the pixel ofinterest for the purpose of contour enhancement in response to adetermination that the differentials between the pixel of interest andthe surrounding pixels do not fall within the predetermined range. Thegenerated correction value is added to the pixel value of the input RGBsignals.

[0028]FIG. 3 is an illustrative drawing for explaining the pixel ofinterest subjected to contour enhancement.

[0029] In the typical output of an image sensor, each pixel does nothave the data indicative of three R, G, and B colors. Rather, each pixelrepresents one pre-assigned color among the RGB colors such as in theRGB Bayer array. Based on the RGB Bayer array, the RGB conversion unit12 performs a process for assigning the three RGB colors to each pixel.Accordingly, the image data subjected to contour enhancement by thecontour enhancing unit 14 has all the three RGB colors already assignedto each pixel.

[0030] In the present invention, a pixel of interest p[x, y] is given,and the four neighboring pixels p[x, y-1], p[x-1, y], p[x+1, y], andp[x, y+1] are taken into consideration to perform contour enhancement inrespect of the pixel of interest.

[0031] Further, the present invention does not perform uniform contourenhancement unvaryingly over the entirety of image data, but carries outcontour enhancement only on image portions that are identified asneeding contour enhancement according to the determination by thecomparison unit 23. Here, the RGB colors and a luminance component Y arerelated as:

Y=0.299×R+0.587×G+0.114×B.  (1)

[0032] As is shown in this equation, the G component, among the threecolors, contains almost 60% of the total luminance component. When adecision has to be made as to whether contour enhancement needs to beapplied, therefore, only the G component may be referred to, therebyachieving an efficient check.

[0033] In the first embodiment of contour enhancement, the G componentis utilized to determine whether contour enhancement needs to be appliedto a pixel of interest.

[0034] In respect of G[x, y] of a pixel of interest, a differencebetween this G component and an average of G components of thesurrounding pixels is computed. A check is then made as to whether thefollowing condition is satisfied.

low _(—) limit<(G[x−1,y]+G[x+1,y]+G[x,y−1]+G[x,y+1])/4−G[x,y]<max _(—)limit  (2)

[0035] Here, the lower limit low_limit and the upper limit max⁻limit aregenerated by the dead-zone generating unit 22. The differentialcalculating unit 21 generates the difference between the G component ofthe pixel of interest and the average of G components of the surroundingpixels for comparison with the upper and lower limits. The comparisonunit 23 determines whether the condition (2) is satisfied. The lowerlimit low_limit and the upper limit upper_limit may be obtained by useof the values of the surrounding pixels as follows

low ^(—) limit=L _(—) limit×pixel _(—) value+L _(—) offset

max _(—) limit=M _(—) limit×pixel _(—) value−M _(—) offset

[0036] Here, pixel_value may be an average or median of the values ofpredetermined surrounding pixels. L_limit and L_offset are the valuesfor defining the lower limit of the dead zone. M_limit and M_offset arethe values for defining the upper limit of the dead zone. L_limit andM_limit may be the same value, or may be set to different values.L_offset and M_offset may be the same value, or may be set to differentvalues.

[0037] When the condition (2) is satisfied, it is decided that contourenhancement is not performed. The enhancement value generating unit 24does not generate a correction value (enhancement value) in this case.As a result, the pixel of interest of the input signals will have thesame R, G, and B color values as before, i.e.,

R[x,y]=R[x,y]

G[x,y]=G[x,y]

B[x,y]=B[x,y].

[0038] When the condition (2) is not satisfied, i.e., when thedifference between the G component of the pixel of interest and theaverage of G components of the surrounding pixels falls within the rangedefined by the upper and lower limits, the enhancement value generatingunit 24 generates a correction value (enhancement value). The correctionvalue is then added to (or rather “subtracted from” as a form ofexpression) each of the R, G, and B colors of the pixel of interest ofthe input signals.

R[x,y]=R[x,y]−Rgain×((G[x−1,y]+G[x+1,y]+G[x,y−1]+G[x,y+1])/4−G[x,y])

G[x,y]=G[x,y]−Ggain×((G[x−1,y]+G[x +1,y]+G[x,y−1]+G[x,y+1])/4−G[x,y])

B[x,y]=B[x,y]−Bgain×((G[x−1,y]+G[x+1,y]+G[x,y−1]+G[x,y+1])/4−G[x,y])

[0039] Here, the second term on the right-hand side of each equation isthe correction value (enhancement value). Rgain, Ggain, and Bgain aregain factors for defining the degree of contour enhancement with respectto R, G, and B, respectively. It suffices to have the gain factors thatare more than zero, and the gain factors may be set to 3.0, for example.

[0040] In the process as described above, a correction value that isobtained from the G components of the pixel of interest and thesurrounding pixels is applied to another color (e.g., the R color) ofthe pixel of interest for the purpose of enhancing contours of thiscolor (i.g., the R color). It is conceivable that a correction valueobtained from R components of the pixel of interest and the surroundingpixels is used in contour enhancement of the R color. In order toachieve this, however, a memory such as a FIFO memory for 2-line delayneeds to be provided for every single color, resulting in the circuitsize of the differential calculating unit 21 being expanded. In theembodiment described above, correction values for contour enhancementare limited to only the G component, so that the line memory needs to beprovided only for the G component. This configuration requires arelatively small circuit size.

[0041] In the embodiment as described above, contour enhancement is notunvaryingly applied to the entirety of the image data, but is applied toonly those image portions which are identified as requiring contourenhancement. This achieves efficient contour enhancement that avoidsunnecessary enhancement of noise.

[0042] In the following, a variation of the first embodiment of thepresent invention will be described.

[0043] The differential calculating unit 21 calculates differentialsbetween the G component of the pixel of interest and the G components ofthe surrounding pixels as follows.

diff _(—)1=G[x−1,y]−G[x,y]

diff _(—)2=G[x+1,y]−G[x,y]

diff _(—)3=G[x,y−1]−G[x,y]

diff _(—)4=G[x,y+1]−G[x,y]

[0044] Then, low_limit and max_limit are generated by the dead-zonegenerating unit 22 for comparison by the comparison unit 23 as follows.

low _(—) limit<diff _(—)1<max _(—) limit  (3)

low _(—) limit<diff _(—)2<max _(—) limit  (4)

low _(—) limit<diff _(—)3<max _(—) limit  (5)

low _(—) limit<diff _(—)4<max _(—) limit  (6)

[0045] Contour enhancement is not performed if all the conditions (3)through (6) are satisfied. In this case, the enhancement valuegenerating unit 24 does not generate a correction value (enhancementvalue). The values of the pixel of interest of input signals will thusstay the same with respect to each of R, G, and B, i.e.,

R[x,y]=R[x,y]

G[x,y]=G[x,y]

B[x,y]=B[x,y].

[0046] Further, contour enhancement is not performed either if thelargest one of diff_(—)1 through diff_(—)4 has an absolute valueidentical to that of the smallest one of diff_(—)1 through diff_(—)4.This condition corresponds to a border case that comes between a case inwhich the value of the pixel of interest should be increased and a casein which the value of the pixel of interest should be decreased. Becauseof this, no correction is made to the pixel of interest.

[0047] If one of the conditions (3) through (6) is not satisfied,diff_max is identified as having the largest absolute value amongdiff_(—)1 through diff_(—)4, and the enhancement value generating unit24 generates a correction value (enhancement value) as follows. Thegenerated correction value is then added to (or rather “subtracted from”as a form of expression) each of the R, G, and B colors of the pixel ofinterest of the input signals.

R[x,y]=R[x,y]−Rgain×diff _(—) max

G[x,y]=G[x,y]−Ggain×diff _(—) max

B[x,y]=B[x,y]−Bgain×diff _(—) max

[0048] Here, the second term on the right-hand side of each equation isthe correction value (enhancement value). Rgain, Ggain, and Bgain aregain factors for defining the degree of contour enhancement with respectto R, G, and B, respectively. It suffices to have the gain factors thatare more than zero, and the gain factors may be set to 0.8, for example.

[0049] In the variation of the first embodiment described above, aline-delay memory should be provided only for the G component in thesame manner as in the first embodiment, thereby achieving a relativelysmall circuit size. Further, contour enhancement is not unvaryinglyapplied to the entirety of the image data, but is applied to only thoseimage portions which are identified as requiring contour enhancement.This achieves efficient contour enhancement that avoids unnecessaryenhancement of noise. In comparison with the first embodiment, an effectof contour enhancement will be larger since the differential having thelargest absolute value among diff_(—)1 through diff_(—)4 is selected foruse in generating the correction value (enhancement value). At a slopewhere the image level changes monotonously, the first embodiment tendsto generate a small correction value (enhancement value). On the otherhand, the variation of the first embodiment can guarantee a relativelylarge correction value (enhancement value), thereby producing asufficient contour enhancement effect.

[0050]FIG. 4 is a block diagram of the contour enhancing unit 14according to a second embodiment of the present invention.

[0051] The contour enhancing unit 14 of FIG. 4 includes a Y-signalgenerating unit 30, a differential calculating unit 31, a dead-zonegenerating unit 32, a comparison unit 33, and an enhancement valuegenerating unit 34. The Y-signal generating unit 30 generates aluminance signal Y from the input RGB signals based on the equation (1)previously presented. In the second embodiment, contour enhancement isperformed by use of the luminance component Y in place of the Gcomponent.

[0052] The differential calculating unit 31 obtains differentials ofpixel values between a pixel of interest and surrounding pixels. Thedead-zone generating unit 32 obtains a predetermined range of pixelsthat is defined by an upper limit and a lower limit. The comparison unit33 checks whether the differentials supplied from the differentialcalculating unit 31 fall into the predetermined range indicated by thedead-zone generating unit 32. The enhancement value generating unit 34generates a correction value (enhancement value) for correcting thevalue of the pixel of interest for the purpose of contour enhancement inresponse to a determination that the differentials between the pixel ofinterest and the surrounding pixels do not fall within the predeterminedrange. The generated correction value is added to the pixel value of theinput RGB signals.

[0053] In respect of a Y component Y[x, y] of a pixel of interest, adifference between this Y component and an average of Y components ofthe surrounding pixels is computed. A check is then made as to whetherthe following condition is satisfied.

low _(—) limit<(Y[x−1,y]+Y[x+1,y]+Y[x,y−1]+Y[x,y+1])/4−Y[x,y]<max _(—)limit  (7)

[0054] Here, the lower limit low_limit and the upper limit max⁻limit aregenerated by the dead-zone generating unit 32 based on the Y componentsof neighboring pixels. The differential calculating unit 31 generatesthe difference between the Y component of the pixel of interest and theaverage of Y components of the surrounding pixels for comparison withthe upper and lower limits. The comparison unit 33 determines whetherthe condition (7) is satisfied.

[0055] When the condition (7) is satisfied, it is decided that contourenhancement is not performed. The enhancement value generating unit 34does not generate a correction value (enhancement value) in this case.As a result, the pixel of interest of the input signals will have thesame R, G, and B color values as before, i.e.,

R[x,y]=R[x,y]

G[x,y]=G[x,y]

B[x,y]=B[x,y].

[0056] When the condition (7) is not satisfied, i.e., when thedifference between the Y component of the pixel of interest and theaverage of Y components of the surrounding pixels falls within the rangedefined by the upper and lower limits, the enhancement value generatingunit 34 generates a correction value (enhancement value). The correctionvalue is then added to (or rather “subtracted from” as a form ofexpression) each of the R, G, and B colors of the pixel of interest ofthe input signals.

R[x,y]=R[x,y]−Rgain×((Y[x−1,y]+Y[x+1,y]+Y[x,y−1]+Y[x,y+1])/4−Y[x,y])

G[x,y]=G[x,y]−Ggain×((Y[x−1,y]+Y[x+1,y]+Y[x,y−1]+Y[x,y+1])/4−Y[x,y])

B[x,y]=B[x,y]−Bgain×((Y[x−1,y]+Y[x+1,y]+Y[x,y−1]+Y[x,y+1])/4−Y[x,y])

[0057] Here, the second term on the right-hand side of each equation isthe correction value (enhancement value). Rgain, Ggain, and Bgain aregain factors for defining the degree of contour enhancement with respectto R, G, and B, respectively. It suffices to have the gain factors thatare more than zero.

[0058] In the process as described above, a correction value that isobtained from the Y components of the pixel of interest and thesurrounding pixels is applied to a given color (e.g., the R color) ofthe pixel of interest for the purpose of enhancing contours of thiscolor (i.g., the R color). With only the Y component being used ingenerating the correction value for contour enhancement, the line delaymemory needs to be provided only for the Y component in the differentialcalculating unit 31. This configuration requires a relatively smallcircuit size.

[0059] In the embodiment as described above, contour enhancement is notunvaryingly applied to the entirety of the image data, but is applied toonly those image portions which are identified as requiring contourenhancement. This achieves efficient contour enhancement that avoidsunnecessary enhancement of noise. Further, this embodiment achievescontour enhancement using accurate luminance information by use of acircuit having a relatively small size.

[0060] In the following, a variation of the second embodiment of thepresent invention will be described.

[0061] The differential calculating unit 31 calculates differentialsbetween the Y component of the pixel of interest and the Y components ofthe surrounding pixels as follows.

diff _(—)1=Y[x−1, y]−Y[x, y]

diff _(—)2=Y[x+1, y]−Y[x, y]

diff _(—)3=Y[x, y−1]−Y[x, y]

diff _(—)4=Y[x, y+1]−Y[x, y]

[0062] Then, low_limit and max_limit are generated by the dead-zonegenerating unit 32 for comparison by the comparison unit 33 as follows.

low _(—) limit<diffy _(—)1<max _(—) limit  (8)

low _(—) limit<diffy _(—)2<max _(—) limit  (9)

low _(—) limit<diffy _(—)3<max _(—) limit  (10)

low _(—) limit<diffy _(—)4<max_limit  (11)

[0063] Contour enhancement is not performed if all the conditions (8)through (11) are satisfied. In this case, the enhancement valuegenerating unit 34 does not generate a correction value (enhancementvalue). The values of the pixel of interest of input signals will thusstay the same with respect to each of R, G, and B, i.e.,

R[x,y]=R[x,y]

G[x,y]=G[x,y]

B[x,y]=B[x,y].

[0064] Further, contour enhancement is not performed either if thelargest one of diffy_1 through diffy_(—)4 has an absolute valueidentical to that of the smallest one of diffy_1 through diffy_(—)4.This condition corresponds to a border case that comes between a case inwhich the value of the pixel of interest should be increased and a casein which the value of the pixel of interest should be decreased. Becauseof this, no correction is made to the pixel of interest.

[0065] If one of the conditions (8) through (11) is not satisfied,diffy_max is identified as having the largest absolute value amongdiffy_(—)1 through diffy_(—)4, and the enhancement value generating unit34 generates a correction value (enhancement value) as follows. Thegenerated correction value is then added to (or rather “subtracted from”as a form of expression) each of the R, G, and B colors of the pixel ofinterest of the input signals.

R[x,y]=R[x,y]−Rgain×diffy _(—) max

G[x,y]=G[x,y]−Ggain×diffy _(—) max

B[x,y]=B[x,y]−Bgain×diffy _(—) max

[0066] Here, the second term on the right-hand side of each equation isthe correction value (enhancement value). Rgain, Ggain, and Bgain aregain factors for defining the degree of contour enhancement with respectto R, G, and B, respectively. It suffices to have the gain factors thatare more than zero, and the gain factors may be set to 0.8, for example.

[0067] In the variation of the second embodiment described above, aline-delay memory should be provided only for the Y component in thesame manner as in the second embodiment, thereby achieving a relativelysmall circuit size. Further, contour enhancement using accurateluminance information is attained. In comparison with the secondembodiment, the variation of the second embodiment can guarantee arelatively large correction value (enhancement value), thereby producinga sufficient contour enhancement effect.

[0068] Further, the present invention is not limited to theseembodiments, but various variations and modifications may be madewithout departing from the scope of the present invention.

What is claimed is:
 1. A semiconductor integrated circuit, comprising: adifferential calculating unit which obtains a differential between avalue of a pixel of interest and values of surrounding pixels containedin an image signal supplied from an image sensor; a dead-zone generatingunit which defines a predetermined range of pixel values; and acomparison unit which checks whether the differential falls outside thepredetermined range, wherein contour enhancement is applied to the pixelof interest in response to a determination by the comparison unit thatthe differential falls outside the predetermined range.
 2. Thesemiconductor integrated circuit as claimed in claim 1, furthercomprising a enhancement value generating unit which obtains anenhancement value based on differentials between the value of the pixelof interest and the values of the surrounding pixels, said enhancementvalue generating unit adding the enhancement value to the value of thepixel of interest in response to the determination by the comparisonunit that the differential falls outside the predetermined range.
 3. Thesemiconductor integrated circuit as claimed in claim 1, wherein saiddifferential calculating unit obtains the differential by using only agreen-color component among a plurality of color components of the imagesignal.
 4. The semiconductor integrated circuit as claimed in claim 1,wherein said differential calculating unit obtains the differential byusing only a green-color component among a plurality of color componentsof the image signal, and said enhancement value generating unit obtainsthe enhancement value by using only the green-color component.
 5. Thesemiconductor integrated circuit as claimed in claim 1, furthercomprising a luminance signal generating unit which obtains a luminancecomponent from a plurality of color components of the image signal, andsaid differential calculating unit obtains the differential by usingonly the luminance component.
 6. The semiconductor integrated circuit asclaimed in claim 1, further comprising a luminance signal generatingunit which obtains a luminance component from a plurality of colorcomponents of the image signal, wherein said differential calculatingunit obtains the differential by using only the luminance component, andsaid enhancement value generating unit obtains the enhancement value byusing only the luminance component.
 7. The semiconductor integratedcircuit as claimed in claim 1, wherein said differential calculatingunit obtains a difference between the value of the pixel of interest andan average of the values of the surrounding pixels as said differential.8. The semiconductor integrated circuit as claimed in claim 1, whereinsaid differential calculating unit obtains a difference between thevalue of the pixel of interest and a value of an adjacent pixel withrespect to each of four neighboring pixels as said differential, and thecomparison unit checks whether the differential falls within thepredetermined range with respect to each of the four neighboring pixels.9. The semiconductor integrated circuit as claimed in claim 8, whereinsaid enhancement value generating unit selects a differential having alargest absolute value among each said differential corresponding to thefour neighboring pixels, and performs said contour enhancement inresponse to size of the differential having the largest absolute value.10. The semiconductor integrated circuit as claimed in claim 9, whereinsaid contour enhancement is not performed if an absolute value of alargest one of each said differential corresponding to the fourneighboring pixels is identical to an absolute value of a smallest oneof each said differential corresponding to the four neighboring pixels.11. A method of enhancing contours, comprising the steps of: obtaining adifferential between a value of a pixel of interest and values ofsurrounding pixels contained in an image signal supplied from an imagesensor; defining a predetermined range of pixel values; checking whetherthe differential falls outside the predetermined range; and applyingcontour enhancement to the pixel of interest in response to adetermination that the differential falls outside the predeterminedrange.